Solid-Oxide Fuel Cell (SOFC) systems operate efficiently by converting the energy contained in a fuel stream into usable heat and electricity. Heat recovery in SOFC systems is commonly achieved by the use of heat exchangers (for example, shell and tube, plate and fin and micro channel) in a variety of flow configurations (multi-stream, counter flow, co-flow, cross flow). Some of the fluid streams in the system are liquid and some are gas, of which some contain water vapour. The SOFC system efficiency can be derived from comparing the potential energy contained in the fuel stream entering the system to the total usable heat and power produced by the system and available to the user. Overall SOFC system efficiency is important as it impacts on the commercial viability of the SOFC, system product in the chosen application market. Thermal and electrical losses are important in determining the overall SOFC system efficiency. Management of heat and mass transfer within the SOFC system influences the magnitude of thermal and electrical losses.
A SOFC itself operates by converting the energy in the fuel into heat and electricity using an electrochemical process. The efficiency of that process is dependent on several factors, including the concentration of the fuel on the fuel side of the fuel cell, partial pressure of oxygen on the air side of the fuel cell, and the temperature of the fuel cell.
In order to operate, the fuel cell consumes fuel and consumes oxygen in the air in the electrochemical reaction. In addition to the electricity generated by this reaction, excess thermal energy is created by the electrochemical process in the region of the fuel cell active area. To maintain the energy conversion process, fuel and air need to be supplied to the fuel cell and heat needs to be removed from the fuel cell. Commonly, the heat generated by the fuel cell reaction is partly consumed keeping the fuel cell itself and its surrounding environment at operating temperature, and the majority of the remaining heat is removed from the fuel cell using the air stream, and/or the fuel exhaust stream.
In general, an operating fuel cell does not consume all the fuel in the fuel stream, and likewise does not consume all the oxygen in the air stream. As the fuel cell does not fully consume the fuel and the oxygen in air, there must be a method of removing the depleted fuel stream (commonly termed the anode off-gas) and the altered air stream (commonly called the cathode off-gas) from the fuel cell active area. Thus, fuel is fed to and removed from the fuel cell active area, and air is fed to and removed from the fuel cell active area.
To achieve effective energy conversion by the fuel cell, a fuel reformer may be included in the fuel supply line before the fuel cell or reformation may occur internally to the fuel cell stack, to reform the hydrocarbon based fuel into a hydrogen rich stream before the fuel gas reached the fuel cell.
SOFCs operate effectively at a specified operating temperature, often over a range of temperatures around the specified operating temperature. This effective operating temperature is typically set by the type of material used in the fuel cell active layers—e.g. 720-950° C. for YSZ, 500-650° C. for CGO.
For SOFCs, the incoming air and fuel streams may be heated to about the fuel cell operating temperature before the streams reach the fuel cell active area. This improves the operating efficiency of the fuel cell and reduces the temperature gradients, and hence thermal stresses, that the fuel cell would undergo if ambient temperature fluids were to come in contact with the hot 500-900° C. fuel cell structures. Efficiency is also improved due to improved thermal balancing of the cell to the optimal operating temperature range. Because of the high operating temperatures, the fuel stream is normally a gas at or close to the point where it meets the fuel cell active area.
Heat energy may be extracted form the gas streams exiting the fuel cell active area (anode and cathode off-gasses) and used to heat the fuel and air streams entering into the fuel cell active area. This is generally achieved by mixing the fuel cell exhaust fuel stream (which contains chemical energy in the form of unused fuel) and the fuel cell exhaust air stream and burning the resultant mix very close to the fuel cell stack (as shown in U.S. Pat. No. 5,212,023 and EP1037296) and using the heat generated by this process to pass, via a heat exchanger, to the incoming air stream.
When fuel cell systems are fed with hydrocarbon fuels it is not uncommon for a fuel reformer to be placed in the fuel stream ahead of the fuel cell stack in order to facilitate the reformation of the hydrocarbon fuel into constituent parts: hydrogen, carbon dioxide, carbon monoxide and other elements. There are several reforming methods suitable for fuel cell use which are known and are thus are not detailed herein. Typical reforming methods include auto-thermal reforming (ATR), steam reforming (SR), water gas-shift reforming (WGS) and partial oxidation reforming (POX or CPOX).
In overview, for effective reformer operation, there are some methods of reforming that do not require water to be added to the fuel stream to operate (e.g. CPOX), and there are those that do require water to be added (e.g. ATR, SR, WGS).
Non-water reformer types, such as CPOX, do not require a water supply unit to be part of the fuel cell system. To one skilled in the art, it will be understood that such non-water added systems produce a lower hydrogen concentration in the reformed fuel stream than is provided from water added systems, which produce a richer hydrogen fuel stream.
For optimum fuel cell operating efficiency, the reforming options using steam offer considerably greater gains in the potential operating efficiency. In systems seeking such efficiency, water is added in the system to produce steam. The steam can be provided from the water content in the fuel side exhaust stream and/or from a water store or water source. At system start up from ambient temperature, there may not be steam directly available in the system from the fuel side exhaust stream, and thus the steam may be generated from a water store using a steam generator.
In some applications, there is a requirement to heat an external load—for instance a hot water store. Thus some of the heat generated by the SOFC system can be used to provide for this heating requirement.